On the Zn side, the local change of the pH alters the composition of the discharge product. The cell can operate until the surface of the carbon cathode is fully covered by electronically insulating LiCl and S discharge products. It follows a similar discharge reaction where the reaction product is LiS 2 O 4. Figure 19 Depiction of the components of a lead acid battery showing the differences between theoretical and practical energy density of a lead acid battery and source of the differences.
Reprinted with permission from a brochure by Sony Corporation. Figure 21 Summary of the reactions and processes that occur in the various fuel cell systems.
Figure 23 Depiction of the components of a complete fuel cell system including the re-former and power conditioning unit.
Figure 24 Typical power curve for a fuel cell. The voltage drops quickly from the OCV due to the formation of the peroxide intermediate. Operation of the fuel cell at the knee of the curve where concentration is limiting performance can damage the electrodes and lead to rapid deterioration of cell operation.
Figure 25 A Comparison of the energy storage capability of fuel cells and batteries. B Fuel cells have a set volume and weight for the fuel cell stack and peripherals to supply the reactants to the stack. The small incremental fuel volume to continue operation supplying energy makes them more efficient for longer operations.
Figure 26 Schematic of a polymer electrolyte membrane PEM fuel cell. Fuel options include pure hydrogen, methanol, natural gas, and gasoline. Figure 28 A, top Simple Helmholtz model of the electrical double layer. It is essentially a picture of a conventional capacitor. The inner Helmholtz plane IHP refers to the distance of closest approach of specifically adsorbed ions and solvent molecules to the electrode surface. The outer Helmholtz plane OHP refers to the distance of ions, which are oriented at the interface by coulomb forces.
His fields of specialization are applied electrochemistry, chemical technology and solid state electrochemistry with special emphasis on the development and characterization of novel materials for rechargeable lithium batteries. Ralph J. Brodd is President of Broddarp of Nevada.
He has over 40 years of experience in the technology and market aspects of the electrochemical energy conversion business. His experience includes all major battery systems, fuel cells, and electrochemical capacitors. View the notice. View Author Information. Broddarp of Nevada, Inc. Cite this: Chem. Article Views Altmetric -. Citations Systems for electrochemical energy storage and conversion include batteries, fuel cells, and electrochemical capacitors ECs.
Figures 1 and 2 show the basic operation mechanisms of the three systems. Note that batteries, fuel cells, and supercapacitors all consist of two electrodes in contact with an electrolyte solution. The requirements on electron and ion conduction in electrodes and the electrolyte are given in Figure 1 and are valid for all three systems. High Resolution Image. In batteries and fuel cells, electrical energy is generated by conversion of chemical energy via redox reactions at the anode and cathode.
As reactions at the anode usually take place at lower electrode potentials than at the cathode, the terms negative and positive electrode indicated as minus and plus poles are used. The more negative electrode is designated the anode, whereas the cathode is the more positive one.
The difference between batteries and fuel cells is related to the locations of energy storage and conversion. In other words, energy storage and conversion occur in the same compartment. Fuel cells are open systems where the anode and cathode are just charge-transfer media and the active masses undergoing the redox reaction are delivered from outside the cell, either from the environment, for example, oxygen from air, or from a tank, for example, fuels such as hydrogen and hydrocarbons.
Energy storage in the tank and energy conversion in the fuel cell are thus locally separated. In electrochemical capacitors or supercapacitors , energy may not be delivered via redox reactions and, thus the use of the terms anode and cathode may not be appropriate but are in common usage. In comparison to supercapacitors and fuel cells, batteries have found by far the most application markets and have an established market position. Fuel cells established their usefulness in space applications with the advent of the Gemini and Apollo space programs.
The most promising future markets for fuel cells and supercapacitors are in the same application sector as batteries. In other words, supercapacitor and fuel cell development aim to compete with, or even to replace, batteries in several application areas. Thus, fuel cells, which originally were intended to replace combustion engines and combustion power sources due to possible higher energy conversion efficiencies and lower environmental impacts, are now under development to replace batteries to power cellular telephones and notebook computers and for stationary energy storage.
The motivation for fuel cells to enter the battery market is simple. To compare the power and energy capabilities, a representation known as the Ragone plot or diagram has been developed. A simplified Ragone plot Figure 3 discloses that fuel cells can be considered to be high-energy systems, whereas supercapacitors are considered to be high-power systems.
Batteries have intermediate power and energy characteristics. There is some overlap in energy and power of supercapacitors, or fuel cells, with batteries. Indeed, batteries with thin film electrodes exhibit power characteristics similar to those of supercapacitors. Finally, Figure 3 also shows that no single electrochemical power source can match the characteristics of the internal combustion engine. High power and high energy and thus a competitive behavior in comparison to combustion engines and turbines can best be achieved when the available electrochemical power systems are combined.
The inferiority of batteries is evident. Figure 5 , showing driving ranges of battery-powered cars in comparison to a cars powered by a modern combustion engine, gives an impressive example of why fuel cells, and not batteries, are considered for replacement of combustion engines. The theoretical values in Figure 4 are an indication for the maximum energy content of certain chemistries. However, the practical values differ and are significantly lower than the theoretical values.
The difference between the theoretical and practical energy storage capabilities is related to several factors, including 1 inert parts of the system such as conductive diluents, current collectors, containers, etc.
However, as batteries and fuel cells are not subject to the Carnot cycle limitations, they may operate with much higher efficiencies than combustion engines and related devices. The following definitions are used during the course of discussions on batteries, fuel cells, and electrochemical capacitors. A primary battery is a cell, or group of cells, for the generation of electrical energy intended to be used until exhausted and then discarded.
Primary batteries are assembled in the charged state; discharge is the primary process during operation. A secondary battery is a cell or group of cells for the generation of electrical energy in which the cell, after being discharged, may be restored to its original charged condition by an electric current flowing in the direction opposite to the flow of current when the cell was discharged.
Other terms for this type of battery are rechargeable battery or accumulator. As secondary batteries are ususally assembled in the discharged state, they have to be charged first before they can undergo discharge in a secondary process.
A specialty battery is a primary battery that is in limited production for a specific end-use. In this paper specialty batteries will not be particularly addressed. The anode is the negative electrode of a cell associated with oxidative chemical reactions that release electrons into the external circuit. The cathode is the positive electrode of a cell associated with reductive chemical reactions that gain electrons from the external circuit. Active mass is the material that generates electrical current by means of a chemical reaction within the battery.
An electrolyte is a material that provides pure ionic conductivity between the positive and negative electrodes of a cell. A separator is a physical barrier between the positive and negative electrodes incorporated into most cell designs to prevent electrical shorting.
The separator can be a gelled electrolyte or a microporous plastic film or other porous inert material filled with electrolyte. Separators must be permeable to the ions and inert in the battery environment. A fuel cell is an electrochemical conversion device that has a continuous supply of fuel such as hydrogen, natural gas, or methanol and an oxidant such as oxygen, air, or hydrogen peroxide.
It can have auxiliary parts to feed the device with reactants as well as a battery to supply energy for start-up.
An electrochemical capacito r is a device that stores electrical energy in the electrical double layer that forms at the interface between an electrolytic solution and an electronic conductor. Open-circuit voltage is the voltage across the terminals of a cell or battery when no external current flows. It is usually close to the thermodynamic voltage for the system. Closed-circuit voltage is the voltage of a cell or battery when the battery is producing current into the external circuit.
Discharge is an operation in which a battery delivers electrical energy to an external load. Charge is an operation in which the battery is restored to its original charged condition by reversal of the current flow. Internal resistance or impedance is the resistance or impedance that a battery or cell offers to current flow. Thermal runaway is an event that occurs when the battery electrode's reaction with the electrolyte becomes self-sustaining and the reactions enter an autocatalytic mode.
This situation is responsible for many safety incidents and fires associated with battery operations. The energy storage and power characteristics of electrochemical energy conversion systems follow directly from the thermodynamic and kinetic formulations for chemical reactions as adapted to electrochemical reactions. First, the basic thermodynamic considerations are treated. The voltage of the cell is unique for each reaction couple.
The amount of electricity produced, nF , is determined by the total amount of materials available for reaction and can be thought of as a capacity factor; the cell voltage can be considered to be an intensity factor. The usual thermodynamic calculations on the effect of temperature, pressure, etc.
Spontaneous processes have a negative free energy and a positive emf with the reaction written in a reversible fashion, which goes in the forward direction. The van't Hoff isotherm identifies the free energy relationship for bulk chemical reactions as where R is the gas constant, T the absolute temperature, A P the activity product of the products and A R the activity product of the reactants.
Combining eqs 4 and 5 with the van't Hoff isotherm, we have the Nernst equation for electrochemical reactions: Faraday's laws, as summarized in eq 7, give the direct relationship between the amount of reaction and the current flow. There are no known exceptions to Faraday's laws. Assuming thermodynamic reversibility 2 of the cell reaction and with the help of eqs 1 and 3, we can obtain the reversible heat effect. By measuring the cell voltage as a function of temperature, the various thermodynamic quantities for the materials in an electrode reaction can be determined experimentally.
Heating and cooling of the cell can proceed with heat exchange with the environment. In general, the entropic heat is negligibly small compared to the irreversible heat released, q , when a cell is in operation.
Irreversible behavior manifests itself as a departure from the equilibrium or thermodynamic voltage. In this situation, the heat, q , given off by the system is expressed by an equation in which E T is the practical cell terminal voltage and E OCV is the voltage of the cell on open circuit.
The total heat released during cell discharge is the sum of the thermodynamic entropy contribution plus the irreversible contribution. This heat is released inside the battery at the reaction site on the surface of the electrode structures. Heat release is not a problem for low-rate applications; however, high-rate batteries must make provisions for heat dissipation.
Thermodynamics describe reactions at equilibrium and the maximum energy release for a given reaction. Electrochemical reaction kinetics follow the same general considerations as those for bulk chemical reactions. The detailed mechanism of battery electrode reactions often involves a series of physical, chemical, and electrochemical steps, including charge-transfer and charge transport reactions.
This type of kinetics is best understood using the absolute reaction rate theory or the transition state theory. In these treatments, the path followed by the reaction proceeds by a route involving an activated complex, where the rate-limiting step is the dissociation of the activated complex. The exchange current density is directly related to the reaction rate constant, to the activities of reactants and products, and to the potential drop across the double layer. Reactions with larger i o are more reversible and have lower polarization for a given current flow.
The activation polarization follows the Tafel equation derived from eq 13 where a and b are constants. Ohmic polarization arises from the resistance of the electrolyte, the conductive diluent, and materials of construction of the electrodes, current collectors, terminals, and contact between particles of the active mass and conductive diluent or from a resistive film on the surface of the electrode.
Concentration polarization arises from limited mass transport capabilities, for example, limited diffusion of active species to and from the electrode surface to replace the reacted material to sustain the reaction. For limited diffusion the electrolyte solution, the concentration polarization, can be expressed as where C is the concentration at the electrode surface and C o is the concentration in the bulk of the solution.
The movement or transport of reactants from the bulk solution to the reaction site at the electrode interface and vice versa is a common feature of all electrode reactions. Most battery electrodes are porous structures in which an interconnected matrix of small solid particles, consisting of both nonconductive and electronically conductive materials, is filled with electrolyte.
Porous electrode structures are used to extend the available surface area and lower the current density for more efficient operation. In practical batteries and fuel cells, the influence of the current rate on the cell voltage is controlled by all three types of polarization. A variety of experimental techniques are used to study electrochemical and battery reactions. The impedance behavior of a battery is another common technique that can reveal a significant amount of information about battery operation characteristics.
A schematic of a battery circuit and the corresponding Argand diagram, illustrating the behavior of the simple electrode processes, are shown in Figure 7 a. Each electrode reaction has a distinctive, characteristic impedance signature. A schematic of a battery circuit and the corresponding Argand diagram, illustrating the behavior of the simple processes, are shown in Figure 7b. Here R is related to the exchange current for the reaction and C is called the polarization capacitance, C P.
Some electrochemical capacitors take advantage of this capacitance to improve their performance of the supercapacitors. Battery electrodes have large surface areas and, therefore, exhibit large capacitances. It is common for cells to have a capacitance of farads and a resistance of milliohms. These techniques provide diagnostic techniques that identify materials properties and materials interactions that limit lifetime, performance, and thermal stabiity.
The accelerated rate calorimeter finds use in identifying safety-related situations that lead to thermal runaway and destruction of the battery.
Most practical electrodes are a complex composite of powders composed of particles of the active material, a conductive diluent usually carbon or metal powder , and a polymer binder to hold the mix together and bond the mix to a conductive current collector.
This yields a much greater surface area for reaction than the geometric area and lowers polarization. The pores of the electrode structures are filled with electrolyte. Although the matrix may have a well-defined planar surface, there is a complex reaction surface extending throughout the volume of the porous electrode, and the effective active surface may be many times the geometric surface area.
Ideally, when a battery produces current, the sites of current production extend uniformly throughout the electrode structure. A nonuniform current distribution introduces an inefficiency and lowers the expected performance from a battery system. In some cases the negative electrode is a metallic element, such as zinc or lithium metal, of sufficient conductivity to require only minimal supporting conductive structures.
Two types of current distribution, primary and secondary, can be distinguished. The primary distribution is controlled by cell geometry. The placing of the current collectors strongly influences primary current distribution on the geometric surface area of the electrodes. The monopolar construction is most common. The differences in current distribution for top connections and opposite end current collection are shown in Figure 8 A,B. With opposite end connections the current distribution is more uniform and results in a more efficient use of the active material.
The bipolar construction depicted in Figure 8C gives uniform current distribution wherein the anode terminal or collector of one cell serves as the current collector and cathode of the next cell in pile configuration.
Secondary current distribution is related to current production sites inside the porous electrode itself. The incorporation of porous electrode structures increases the surface area and shortens diffusion path lengths to the reaction site. Current-producing reactions can penetrate into a porous electrode structure to considerable depth below the surface of the electrodes as noted in Figure 9. The location of the reaction site inside a porous electrode is strongly dependent on the characteristics of the electrode structure and reactions themselves.
The key parameters include the conductivity of the electrode matrix, electrolyte conductivity, the exchange current, the diffusion characteristics of reactants and products, and the total current flow. In addition, the porosity, pore size, and tortuosity of the electrode play a role. Factors that influence the secondary current distribution are the conductivity of the electrolyte and electrode matrix, the exchange current of the reactions, and the thickness of the porous layer.
Sophisticated mathematical models to describe and predict porous electrode performance of practical systems have been developed. These formulations based on models of primary and secondary battery systems permit rapid optimization in the design of new battery configurations. The high-rate performance of the present SLI automotive batteries has evolved directly from coupling current collector designs with the porous electrode compositions identified from modeling studies.
Modeling has become an important tool in developing new battery technology as well as for improving the performance of existing commercial systems. Models based on engineering principles of current distribution and fundamental electrochemical reaction parameters can predict the behavior of porous electrode structures from the older lead acid automotive technology to the newest lithium ion Li ion technology. Batteries are self-contained units that store chemical energy and, on demand, convert it directly into electrical energy to power a variety of applications.
The latter are mainly military and medical batteries that do not find wide commercial use for various reasons of cost, environmental issues, and limited market application. They generally do not require time to start-up. Success in the battery market depends largely on four factors, noted in Figure The market for batteries in Table 1 is directly related to the applications they serve, such as automobiles, cellular phones, notebook computers, and other portable electronic devices.
The growth in any particular segment follows closely the introduction of new devices powered by batteries.
The introduction of new materials with higher performance parameters gives the various designers freedom to incorporate new functionality in present products or to create new products to expand the market scope. Batteries for notebook computers have experienced double-digit growth, whereas the automobile SLI market segment has grown with the gross national product. Batteries can range in size from aspirin tablet and even smaller with a few tens of mAh, for in-the-ear hearing aids, to a building with 40 MWh for energy storage and emergency power.
Table 1. Figure 11 depicts the basic elements of a battery. Figure 12 illustrates the operation of a battery, showing the energy levels at the anode negative and cathode positive poles and the electrolyte expressed in electronvolts. The negative electrode is a good reducing agent electron donor such as lithium, zinc, or lead. The positive electrode is an electron acceptor such as lithium cobalt oxide, manganese dioxide, or lead oxide. The electrolyte is a pure ionic conductor that physically separates the anode from the cathode.
In practice, a porous electrically insulating material containing the electrolyte is often placed between the anode and cathode to prevent the anode from directly contacting the cathode. Should the anode and cathode physically touch, the battery will be shorted and its full energy released as heat inside the battery.
Battery electrolytes are usually liquid solvent-based and can be subdivided into aqueous, nonaqueous, and solid electrolytes. Aqueous electrolytes are generally salts of strong acids and bases and are completely dissociated in solution into positive and negative ions. The electrolyte provides an ionic conduction path as well as a physical separation of the positive and negative electrodes needed for electrochemical cell operation.
Each electrolyte is stable only within certain voltage ranges. Exceeding the electrochemical stability window results in its decomposition.
The voltage stability range depends on the electrolyte composition and its purity level. The high conductivity of aqueous solvent-based electrolytes is due to their dielectric constants, which favor stable ionic species, and the high solvating power, which favors formation of hydrogen bridge bonds and allows the unique Grotthus conductivity mechanism for protons.
Thermodynamically, aqueous electrolytes show an electrochemical stability window of 1. Compared to water, most organic solvents have a lower solvating power and a lower dielectric constant. This favors ion pair formation, even at low salt concentration. Ion pair formation lowers the conductivity as the ions are no longer free and bound to each other.
Organic electrolytes show lower conductivities and much higher viscosities than aqueous electrolytes. Exceeding the voltage limit in the organic electrolytes results in polymerization or decomposition of the solvent system.
Solid electrolyte batteries have found limited use as the power source for heart pacemakers and for use in military applications.
The basic principles described above apply to fuel cells and electrochemical capacitors as well as to batteries. A list of common commercial systems is found in Table 2. A graphical representation of the energy storage capability of common types of primary and secondary batteries is shown in Figures 13 and It is beyond the scope of this paper to discuss all systems in detail.
Instead, we want to review the most common electrode mechanisms for discharge and charge depicted in Figure Figure 14 Energy storage capability of common rechargeable battery systems. Table 2. Common Commercial Battery Systems. During the cell reaction, Cu is displaced by Li and segregates into a distinct solid phase in the cathode. The products of this displacement type of reaction, Li 2 S and Cu, are stable, and the reaction cannot be easily reversed.
Hence, the electrode reactions cannot be recharged and the cell is considered to be a primary cell, as the discharge reaction is not reversible. The Li electrode in Figure 15B is discharged by oxidation.
The reaction is reversible by redeposition of the lithium. However, like many other metals in batteries, the redeposition of the Li is not smooth, but rough, mossy, and dendritic, which may result in serious safety problems. This is in contrast to the situation with a lead electrode in Figure 15C, which shows a similar solution electrode. Figure 15D shows a typical electrochemical insertion reaction. Unlike displacement type electrodes Figure 15A and solution type electrodes Figure 15B , the insertion electrodes Figure 15D have the capability for high reversibility, due to a beneficial combination of structure and shape stability.
Many secondary batteries rely on insertion electrodes for the anode and cathode. A prerequisite for a good insertion electrode is electronic and ionic conductivity. However, in those materials with poor electronic conductivity, such as MnO 2 , good battery operation is possible.
In this case, highly conductive additives such as carbon are incorporated in the electrode matrix, as in Figure 15E. The utilization of the MnO 2 starts at the surface, which is in contact with the conductive additive and continues from this site throughout the bulk of the MnO 2 particle.
Most electrodes in batteries follow one of the basic mechanisms discussed in Figure Zinc manganese batteries consist of MnO 2 , a proton insertion cathode cf. Figure 15E , and a Zn anode of the solution type. The discharge reaction of the MnO 2 electrode proceeds in two one-electron reduction steps as shown in the discharge curve Figure Starting at cell voltages of 1.
This is consistent with the Gibbs phase rule that predicts the shape of the discharge curve for one- and two-phase reactions Figure If the values of two parameters, usually pressure, p , and temperature, T , are specified, there is no degree of freedom left and other parameters of the system such as voltage have to be constant.
Hence, the cell voltage stays constant for a two-phase discharge reaction. If there is a degree of freedom left, as in the case of a one-phase reaction, the cell voltage can be a variable and changes slopes-off during discharge.
The detailed discharge reaction mechanism is shown in Figure It should be noted that local pH changes occur also during discharge of the MnO 2 electrode Figure 15E. The current version of the alkaline cell is mercury free. Instead, it uses a combination of alloying agents and corrosion inhibitors to lower the hydrogen gas generation from corrosion of the zinc anode and to compensate for the corrosion protection originally provided by the mercury.
A synthetic gel holds the zinc powder anode together. The high energy density results from the cell design, as only the zinc powder anode is contained in the cell. The other reactant, oxygen, is available from the surrounding air. The air electrode is a polymer-bonded carbon, sometimes catalyzed with manganese dioxide. The electrode has a construction similar to that of fuel cell electrodes see section 3.
Many people get confused by the difference between a battery and a fuel cell. Both can be used as sources of power — but in different ways. The electrical energy contained within a battery is either from the factory where it was made, or from charging the battery via an outlet. If your battery dies, you are dependent on either being near a source of electricity to re-charge, or near a store to buy a new one. A fuel cell is different. It takes an energy source, such as propane, diesel or natural gas, and converts it into electrical energy.
As long as you have access to your energy source, you have access to electricity any time you need it — wherever you may be. Whether you are at sea, out camping, in an emergency situation or when the neighborhood power goes out, you can use a fuel cell to create your own electricity. Some people have back-up generators for emergency situations.
Besides, fuel cells are planted in various places such as utility power plants, hospitals, schools, hotels, etc. A battery is a device containing two or more electrochemical cells that can convert chemical energy into electrical energy directly. It has external connections to power electric devices such as flashlights, mobile phones, etc. The electrons move from the negative terminal to the positive terminal through an external circuit. When we connect the battery with an external electric load, a redox reaction takes place.
The reaction can convert high energy reactants to low energy products. Here, the difference between these energy values is delivered to the external circuit in the form of electrical energy. There are two different batteries as primary and secondary batteries. Primary batteries cannot be recharged, but secondary batteries are rechargeable. Furthermore, a fuel cell is continuously supplied with fuel and oxygen from an external source, which makes it work for a long time period; however, a battery contains a limited amount of fuel and oxidant, and these two components decrease with time, so this device cannot supply electrical energy for a long period of time.
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